1. Field of the Invention
The present invention relates to a filter device made up of a piezo-resonator and a branching filter using the same.
2. Description of the Related Art
In recent years, a mobile telecommunication terminal including a portable cellular phone as a typical example is rapidly developing. It is desirable that, in order to meet a demand for its portability, au& the terminal is small in size, lightweight, and power-thrifty in particular. To make such the telecommunication terminal be small in size, lightweight, and power-thrifty, it is necessary that a filter device being used for the telecommunication terminal is also small in size, lightweight, and low in power loss. Moreover, to satisfy a demand for high-speed and large-capacity communications, it is also required that such the filter device has a wide passing bandwidth.
In response to this request, an SAW (Surface Acoustic Wave) filter and a filter device using the SAW filter are widely used.
In the SAW filter, a cross finger-shaped electrode in which electrode fingers each having a width being about one fourth of a wavelength of a surface acoustic wave to be propagated are alternately arranged is formed on a piezoelectric substrate so that the surface acoustic wave is received in an excited state. A line width of an electrode finger of the SAW filter being used in a system operating in a frequency band of 2 GHz is about 0.4 μm. In order to be used in a system operating in a higher frequency, an electrode having its line width of 0.4 μm or less has to be fabricated with high accuracy, which, however, has a probability that manufacturability is remarkably lowered. Therefore, it is thought that, achievement of operations in a further higher frequency by using the SAW filter has a difficulty.
In such the circumstance, a filter device using a piezoelectric thin film being one type of a device operating by employing a BAW (Bulk Acoustic Wave) filter comes into wide use. An operating frequency of the BAW device is determined by a thickness of a piezoelectric film being sandwiched between input/output electrodes. Since ceramic or quarts is conventionally used as a material for the piezoelectric film, it was difficult to fabricate the thin piezoelectric film with high accuracy using such the material and, therefore, the filter using the piezoelectric film has not been used in high frequency applications. However, in recent years, it is made possible to form a piezoelectric film by a depositing apparatus using a sputtering method or a like and, as a result, a piezoelectric film having a desired thickness can be obtained and the BAW device has now come to hold superiority in use in the high frequency applications. Moreover, an electrode that uses the RAW device serving as a thin film resonator filter is a plate electrode in which, unlike in the case of the SAW filter, use of a slender electrode is not required, thus making it possible to process signals having large power.
The conventional BAW device is constructed by forming one layer of a piezoelectric film on a substrate and arranging resonators in which the piezoelectric film is sandwiched between upper and lower electrodes so as to be plane and then electrically connecting each of the resonators by a ladder-connection method.
Moreover, technology to achieve a wide passing bandwidth in the ladder-type filter is disclosed in Japanese Patent Application Laid-open No. Hei 10-93375 in which an inductance element is connected to a parallel arm or in which a difference is given between a resonant frequency of each of resonators formed on a serial arm and an anti-resonant frequency of each of resonators formed on a parallel arm.
However, the conventional filter device has problems. That is, since the filter device generally uses 5 to 7 pieces of resonators, a chip area being no more than an area obtained by multiplying the number of resonators by a resonator area cannot be realized in the filter device having configurations described above.
Moreover, as the BAW device, an SCF (Stacked Crystals Filter) in which a resonator having two or more stacked piezoelectric films is arranged stereoscopically and/or a CRF (Coupled Resonator Filter) are known. The SCF, since it is a filter that operates in a narrow band, is not suitable for a filter that should operate in a wide band. The CRF, since it uses multiple modes, is suitable for a filter that should operate in a wide band, however, it suffers larger insertion loss due to its operational principles when compared with the case of the conventional ladder-type filter and, therefore, is not suitable for a filter that should be low in power loss.
Furthermore, when the ladder-type filter is to be realized by using a layer-stacked structure, since there is a bar that a characteristic is degraded due to mutual interference between serial-arm resonators or parallel-arm resonators being arranged in a layer-stacking direction, this fear must be removed when the ladder-type filter is constructed. If so, an advantage of using the layer-stacked structure is lost in the end.